The present invention relates to electrochemical fuel cells and a method of assembling the same. More specifically, the present invention relates to polymer electrolyte membrane fuel cells (xe2x80x9cPEMxe2x80x9d) built on a ribbed substrate with alternating anode and cathode regions.
Electrochemical fuel cells convert fuel and an oxidant to electricity and reaction products. A typical fuel cell consists of a cathode, an anode, and an electrolyte. The electrolyte is sandwiched between the cathode and anode. Fuel, in the form of hydrogen for example, is supplied to the anode where a catalyst, typically platinum, catalyzes the following reaction:
Anode reaction: H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83Equation (1) 
The catalyst helps separate the stable hydrogen fuel into hydrogen ions and two electrons. At the cathode, an oxidant, in the form of oxygen or oxygen containing air, is supplied to the cathode. In order for electricity to be produced, the hydrogen ions and the two electrons must make their way to the cathode. This can be accomplished in the following way. Once the reaction at the anode occurs, the two hydrogen ions act as cations and accordingly migrate through the electrolyte membrane to the cathode. Movement of the two electrons can be facilitated with an external circuit connecting the anode to the cathode, resulting in the production of electricity.
The air that was supplied to the cathode reacts with the hydrogen ions that have crossed the membrane and with the electrons from the external circuit to form liquid water as the reaction product. This reaction, shown by Equation 2, is typically catalyzed by platinum.                                                         Cathode  reaction:                        ⁢                          xe2x80x83                        ⁢                          1              2                        ⁢                          O              2                                +                      2            ⁢                          H              +                                +                      2            ⁢                          e              -                                      →                              H            2                    ⁢          O                                    Equation  (2)            
As can be seen from the foregoing description, the exemplary hydrogen fuel cell can produce electricity and a reaction product, namely water.
In the prior art, fuel cells have been categorized into five types according to the nature of the electrolyte employed in the cell, namely, alkaline, phosphoric acid, molten carbonate, solid oxide and polymer electrolyte. The present invention pertains to polymer electrolyte fuel cells, also known as the proton-exchange-membrane (xe2x80x9cPEMxe2x80x9d) cells. In a PEM cell, the electrolyte is comprised of a thin membrane made of polymer similar to polytetrafluoroethylene (PTFE or Teflon(copyright)) with sulfonic acid groups included in the polymer molecular structure. The sulfonic acid groups are acid ions, which act as an active electrolyte.
In order for an electrolyte membrane to efficiently perform in a fuel cell, it should allow the flow of ions through the membrane to the cathode, while simultaneously prohibiting the stable fuel molecules from migrating to the cathode. The polymers used in PEM cells have the dual attributes of readily conducting hydrogen nuclei (H+ ions or protons) from the anode to the cathode, while effectively blocking the flow of diatomic hydrogen to the cathode. The PEM cell operates in the same manner as was described above with reference to the exemplary hydrogen fuel cell, i.e., hydrogen protons flow through the electrolyte membrane and electrons are passed through an external electrical conductor.
Some of the operational criteria of the different components of a fuel cell are as follows. Hydrogen gas or methanol fuel must be distributed uniformly over the active area of the anode side of the electrolyte membrane. Similarly, oxygen or air must be distributed uniformly over the cathode side of the electrolyte membrane. The electrolyte membrane must be kept moist. A catalyst must be uniformly dispersed over the active area on both sides of the electrolyte membrane in such a manner that each catalyst-particle site is concurrently accessible to the reactant gas, the polymer electrolyte material, and to a third material which forms an electrically conductive path. A means must be provided to collect the electron flow, which is the electrical current, over the entire area of the membrane, and to ensure an uninterrupted electrically conductive flow path from the catalyzed surfaces of the membrane to these current-collector devices. Finally, the channels or chambers containing the reactant gas must be sealed and isolated from one another and from the ambient atmosphere in order to prevent both wasteful loss of the gases and, more importantly, potentially dangerous mixing of the reactants inside the cell.
Assuming that these conditions can be met, fuel cells can be used in a variety of applications. One well known use for fuel cells is to use them as an alternative power source for automobiles or buses. Because a single fuel cell is only capable of producing a voltage in the range of 0.4 to 0.8 volts, many applications require multiple cells to be assembled in series electrically, enabling higher voltages. One problem with using fuel cells in these stacks is that adding additional fuel cells necessarily increases the battery""s overall size. Conventional PEM fuel cell stacks are built with bipolar carbon plates between the cells. Naturally, these plates contribute to the overall size of the fuel cell design. When a fuel cell is used as a stationary power source, the size of the cell may not be an issue. For portable devices, however, the size and weight of the fuel cell is of paramount importance. There is thus a need for compact fuel cells that can meet the power requirements of portable devices such as cellular telephones, laptop computers, breathalyzer devices, personal digital assistants, and the like.
The invention disclosed herein is directed toward a novel structural design for a PEM fuel cell, as well as a novel method of creating an anode and a cathode via a sputtering technique. This invention can be used with hydrogen or direct methanol fuel cells. The geometry, discussed more fully below, allows a design engineer to construct a compact fuel cell useful in portable devices requiring battery power. In addition to facilitating connecting multiple fuel cells together in a layer, the design of this invention allows for the creation of fuel cell stacks. The sputtering disclosed herein is comprised of sputtering thin film catalysts onto ribbed surfaces, thereby creating anodes and cathodes. In order for a high effective surface area for the fuel and oxidant and their respective reactions to be created, a porous catalyst could be used. In addition, the thickness of the catalysts can be chosen in such a way as to support electron conduction and, therefore, to allow the catalyst and the surface upon which it was sputtered to act as an anode and a cathode.